1. Field of the Invention
This invention relates to display devices and systems, and, in particular, to display devices and systems using liquid crystal materials. In its more particular aspects, the present invention relates to matrix display systems having liquid crystal devices at each matrix cross point.
2. Description of the Prior Art
Liquid crystal display devices are well known in the art, generally including a pair of spaced plates having conductive layers deposited on the opposed facing surfaces thereof, and having a layer of liquid crystal material retained between these surfaces. There are a number of different materials which exhibit liquid crystal properties, and there are a number of control mechanisms for causing such materials to assume the different states required of a material for use in practical display devices. See, for example, "Projecting Images With Liquid Crystals," by L. K. Anderson, Bell Laboratories Record, Vol. 52, No. 7, July/August 1974, pp. 222-229, and "Liquid Crystal Displays in Low Power Applications," by J. A. Castellano and G. W. Taylor, IEEE 1974 Intercon Record of Technical Papers, Session 25, Mar. 26-29, 1974, Paper 25/3.
A preferred class of liquid materials having a number of advantageous fabrication and operating characteristics is that including materials exhibiting nematic liquid crystal states. The so-called field-effect mode of operating a nematic liquid crystal cell typically involves the application of an electric field across the conductive layers of the cell, thereby to realign the molecules of liquid crystal material and change the degree of polarization induced by the cell on incident light. Thus, for example, in a field-effect twisted-nematic liquid crystal cell, the no-field polarizing effect of the cells is such as to induce a 90 degree rotation in polarization, while the full-field condition eliminates the polarizing effect completely, i.e., incident light polarization is not rotated to any substantial degree. By placing an external polarizer-analyzer pair with complementary (90 degree rotated) polarization directions about such a cell, the no-field condition is made to correspond to essentially complete transmission through the cell, while the full field condition corresponds to total absorption in the analyzer.
A principal advantage of the nematic field effect liquid crystal display cell is its low voltage, low power operation. Typical field effect operating voltages are from 5 to 7 volts and power consumption is of the order of 10.sup.-.sup.6 watts/cm.sup.2 of display area, i.e., such that a segmented digit of the type used in watches, e.g., can be driven from a single miniature battery for well over a year.
An important limitation of field effect liquid crystal devices in some applications is the relatively long turn-on time. Thus, for example, the time from application of the voltage pulse to generate the required field to the time that the maximum electro-optic effect (e.g., substantially complete untwisting) is achieved is typically 2-10 milliseconds. Another characteristic of field effect liquid crystal devices is that they have no intrinsic memory, i.e., the untwisting field must be applied constantly.
Drive circuitry for liquid crystal displays is somewhat complicated by the above-recited limitations. In particular, the need to refresh field effect twisted nematic liquid crystal display devices has largely militated against any but the more complicated multiplexing and scanning systems. For example, the above-cited Castellano, et al paper describes one multiplexing scheme involving patterns of information-dependent signals having three components to selectively activate a segmented display. Another system described in U.S. Pat. No. 3,840,695 issued Oct. 8, 1974 to A. G. Fischer for use in a television display involves a transistor per display point with associate peripheral scanning.
U.S. Pat. No. 3,835,463 issued Sept. 10, 1974 to Tsukamoto et al describes means for varying X and Y pulse patterns having selectable phases and geometry to control the transparency of a liquid crystal cell, while in U.S. Pat. No. 3,740,717, issued June 19, 1973 to Huener et al., a typical X-Y matrix liquid crystal system using unipolar pulses is described.
Each of the above prior art systems, however, suffers from complexity of circuit arrangements not justified in many applications, and each suffers further from the need to continually refresh the elements defining a display entity with information-bearing signals.